Fine powder of polytetrafluoroethylene (hereinafter referred to as “PTFE”) is produced by coagulating polymer fine particles obtained by a so-called emulsion polymerization in which tetrafluoroethylene (hereinafter referred to as “TFE”) is polymerized in an aqueous medium with use of an emulsifier. It is technically known that PTFE is modified by copolymerizing TFE with a relatively small amount of a comonomer copolymerizable therewith.
It is also known that modification of PTFE is effective for improving processability in paste extrusion of fine powder with an appropriate coagent as an additive.
JP-B-37-4643 describes a polymerization method of a modified PTFE in which a modifier is added to a polymerization system before 70% of a predetermined amount of TFE is consumed, and describes examples of the modifier such as a perfluoroalkyltrifluoroethylene comonomer represented by hexafluoropropylene (hereinafter referred to as “HFP”) and a chain transfer agent represented by methanol.
Furthermore, JP-B-44-14937 proposes use of two kinds of initiators consisting of a persulfate and a disuccinic acid peroxide, in a system containing perfluoroalkyltrifluoroethylene or perfluoroalkyloxytrifluoroethylene as a comonomer. JP-B-56-26242 proposes formation of a core-shell structure using a chlorotrifluoroethylene (hereinafter referred to as “CTFE”) comonomer.
JP-B-56-26243 proposes addition of HFP, ω-hydroperfluoroolefin or vinylidene fluoride after polymerization of 70% of the total amount of monomers.
JP-B-57-18529 proposes formation of a core-shell structure using HFP or ω-hydroperfluoroolefin as a comonomer.
It is known that the modified PTFE fine powder obtained by these methods is excellent in extrudability but low in heat resistance. Therefore, JP-B-04-3765 proposes a core-shell structure composed of a copolymer with a linear-fluoroalkyl or chain-fluoroalkyl vinyl ether comonomer for the core and a copolymer with CTFE for the shell, in order to improve the heat resistance. However, it is insufficient in terms of reliable heat resistance because the copolymer has units based on thermally unstable CTFE.
Moreover, JP-B-03-66926 and JP-B-08-26102 propose methods of modifying PTFE using Rfa-CH═CH2 (Rfa is a perfluoroalkyl group) as a comonomer. They describe the methods of continuously adding the comonomer up to a midway point of the polymerization in order to increase the degree of modification at an early stage. This comonomer is also inadequate to achieve reliable heat resistance because its structure is not a perfluoro-structure but a hydrogen-containing structure. In particular, further improvement in the heat resistance is recently required with increasing demands for the heat resistance of parts due to environmental measures to automobile exhaust emissions.
JP-A-09-87334 proposes an improvement in the heat resistance by making a core-shell structure composed of a copolymer with a perfluorobutylethylene comonomer for the core and a copolymer with a HFP comonomer for the shell (at least 75% of the yield).
In addition, the brochure of WO00/02935 proposes an improvement in the heat resistance by making a core-shell structure composed of a copolymer with a linear-fluoroalkyl or chain-fluoroalkyl vinyl ether comonomer for the core and PTFE obtained with a chain transfer agent added in the polymerization of TFE for the shell (at least 80% of the yield).
Furthermore, JP-A-05-170834 describes a TFE copolymer capable of undergoing melt molding, which is a copolymer of TFE with a slight amount of CF2═CF—O—(CF2)nCF═CF2 (where n is an integer of from 1 to 6). However, it fails to describe whether of not the TFE copolymer can undergo paste extrusion.
U.S. Pat. No. 6,479,591 discloses a TFE copolymer of a core-shell structure in which the core is made by copolymerization of a slight amount of a specific cyclic monomer or ring-forming monomer and in which the shell is made by copolymerization of HFP.
As described above, studies have been made on the use of the binary system of the persulfate and the disuccinic acid peroxide as a polymerization initiator, and on the TFE copolymers with both excellent paste extrudability and excellent heat resistance by the core-shell structure composed of the combinations of various comonomers conventionally known. However, the perfluoro monomer is generally low in reactivity and has a drawback in productivity and, for further improvements in the moldability, transparency, heat resistance, and so on, there have been needs for a comonomer having a novel structure, and for a TFE copolymer of a novel composition using the comonomer.
An object of the present invention is to provide a TFE copolymer and a fine powder thereof with excellent paste extrudability, heat resistance and transparency, which is a TFE copolymer produced by copolymerization of TFE with a comonomer having good copolymerization reactivity with TFE.